The art of remote sensing of objects, for example, by sonar, has been developing for many years. Initially, a single acoustic sensor was towed by or attached to a submarine or surface-going vessel. This acoustic sensor, commonly called a hydrophone, simply notified the operators of the vessel of the presence of an object, and possibly of its distance. This simple system was improved upon by devising sonar systems which included multiple sensors arranged in an array. Originally, these sensors were arranged along a tow line. The increase in the number of sensors dramatically increased the gain available for sonar systems. In addition, having multiple sensors allows a rough determination of the direction of a target. However, a single line of acoustic sensors cannot resolve vertical or xe2x80x9cleft/rightxe2x80x9d ambiguity in the position of the target. Also, overall achievable gain is limited by the strength of the line, and considerations related to towing drag and the ability to predict the coherence of the acoustic wavefront.
The above problems have been resolved by producing what are called xe2x80x9cvolumetric arrays.xe2x80x9d FIG. 1 illustrates such an array. In FIG. 1, vessel 101 drags a tow cable, 102. The tow cable in turn is attached to a multiple-line, three-dimensional, array of acoustic sensors, 103. When the signals from these multiple lines of sensors are combined, the apparent gain is equivalent to the total number of sensors. In addition, the three-dimensional nature of the array allows beam forming and signal processing to be used in order to determine the position of a sensed object with greater accuracy than possible with single-line arrays.
The ability of a volumetric array to sense the position of objects depends on knowledge of the relative location of the elements of the array. The lines of the array may be made of rigid material or put in tension, and the relative location of the elements may be assumed. However, a flexible, free-floating array of lines can be built at lower cost than a rigid array. Such a flexible array also creates less drag, and is less likely to be damaged. It is not necessary that the elements of the array be positioned in any particular fashion. Rather, it is only necessary to know the relative positions of the elements. Therefore, various electronic means have been developed for determining the positions of the various sensors in an array at regular intervals.
Known methods of determining the position of acoustic sensors in a volumetric array suffer from one or more shortcomings. In some cases, the signals which are sent to the array elements for position sensing are within the analysis band of the array. The use of the analysis band for positioning destroys any chance of the vessel which is towing the array remaining concealed, since other vessels towing similar arrays will be able to pick up the sensing pulses. Additionally, the granularity with which positioning can be determined is dependant on the wavelength used for sensing signals. Acoustical waves have long wavelengths and thus limit the accuracy with which sensor position can be determined. Another problem with known methods is the difficulty with supplying multiple transmitters for sensing pulses. Ideally, the more transmitters that are provided, the more accuracy can be achieved. However, it is necessary for the sensors, or receivers, to be able to determine which signal is coming from which transmitter. If different frequencies are used, each transmitter must have a slightly different design, therefore raising the cost of the array. Additionally, sensors and/or receivers in the array which are to receive the sensing pulses, need to be more complex. There is a need for a volumetric array positioning system which can achieve very high accuracy, and which can remain undetected by other vessels when in use. Ideally, the system should also be able to be built out of multiple transmitters and sensors of identical design.
The present invention provides for measurement system for a volumetric sensor array that enables very high accuracy determination of the exact position of the various lines of the array. The system according to the present invention operates well outside the analysis band of the array, thus facilitating a covert process. In the case of a sonar array, ultrasonic frequencies are used, which have not only the advantage of not being targeted by other vessels"" sonar systems, but also the advantage of propagating poorly through seawater. In addition, the transmitters used in the invention can all operate on exactly the same ultrasonic frequency, facilitating the use of many identical transmitters, thus lowering cost as compared to systems where each transmitter must have a unique design. Furthermore, the invention employs code division multiple access (CDMA) type pseudo-random numbers in such a way that the number of transmitters which can be used, and still allow for the unique identification of each transmitter by each receiver, is limited only by mechanical considerations such as the size, weight, and drag of the array.
According to the invention, the array includes at least one, and preferably a plurality of transmitter subsystems and a plurality of sensors. The array also includes detectors, which are associated with the position determining system of the array lines. In the case of a sonar array, the sensors are typically hydrophones. In one embodiment, a shipboard receiver generates a global sync signal common to all array lines in the system. This sync is typically the same as that used in the synchronization of the acoustic channel sampling. The specific PRN sequence that is used in a specific transmitter subsystem is generated locally from the electronics included in each array line of the volumetric system of arrays. The sequence is sent to the receive lines as a synchronization pulse, so that the detectors in the receive lines can acquire the sequence, and determine the timing difference relative to the global sync timing reference. Each specific transmitter transmits its specific PRN sequence by modulating a carrier wave being sent from a transducer for the specific transmitter subsystem. The carrier wave has a frequency, which is outside the acoustic analysis band for the volumetric array. A detector on an array line identifies the specific PRN sequence at a sensor on the line and determines a transit time for the sequence from the specific transmitter to the sensor. At least two sensors are needed. Each sensor can have it""s own detector, or a system could be designed in which a detector servers multiple sensors. The transit time, in increments of clock periods, is then communicated to the shipboard receiver for position determination. Because at least two sensors per line are used, both a distance and angle for the receive line involved can be determined. Also, since each transmitter subsystem has a specific pseudo-random number sequence associated with its transmission, each transmitter in the array can be uniquely identified by any detector. Arrays with many transmitters can be readily built and all transmitters can be identical. In fact, it is possible to build an array in which every line has a transmitter subsystem, detectors, and sensors. Since transmitters are identified by codes, the system works according to a code division multiple access (CDMA) scheme, similar to that used for various types of data communications.
Transit times can be sent to the shipboard receiver in any of various formats. In one embodiment, the transit times to two sensors are encoded as a pair of 16-bit words which represents a transit time from the reference transmitter to the detector pair. The phase is extracted at the shipboard receiver by applying simple trigonometric relationships based on the measured and known. In another embodiment, the transit times are converted to analog levels and sent to the shipboard receiver. In any case, a synchronization block that is inside, or closely associated with, a shipboard receiver generates the timing reference for all of the PRN sequences for all of the transmitters. The sequences can be transmitted continuously, facilitating a constant awareness of the exact positioning of the various array lines and fully exploit the inherent processing gain available in the CDMA technique.
A transmitter subsystem according an embodiment of the invention consists of a transmitter and an omni-directional ceramic transducer. The transmitter modulates the carrier wave using binary phase shifted keying (BPSK). The detectors which are used to determine the transit times and forward those back to the shipboard receiver, demodulate the carrier wave using a phased lock loop (PLL). The carrier wave has a frequency which is outside the analysis band of the array. A demodulator recovers the sequence from the carrier wave and forwards that sequence to a shifter which is connected to the demodulator and clocks out the pseudo-random number sequence to a sequence detection block. The sequence detection block within the detector identifies the specific PRN sequence from the specific transmitter by comparing the sequence received from the demodulator with the sequence received during the synchronization pulse. A counter, which began running when the global synchronization pulse is received, is latched when the sequence is detected by the sequence detection block. Finally, an accumulator is connected to the counters which keep a transit time count for each sensor. The accumulator puts the transit times into an appropriate format and communicates the transit times to the shipboard receiver. The synchronization block, the transmitter subsystems, and the detectors, together with the required sensors and transducers, all work together to form the means to carry out the method of the invention.